More work to getting stippling working.

Now an image can be selected. Found a way to convert it easily to
grayscale. The canvases now resize to the size of the selected image.

Currently not working as intended, some bug is occuring that needs to be
sorted out.

Worked on #3, #5, #9

	new file:   filters.svg
	modified:   index.html
	modified:   voronoi.js